Extracorporeal processing of blood, either to remove certain disease-contributing constituents or to obtain specific fractions for subsequent therapeutic use, is becoming increasingly important in medicine. The term "pheresis" generally is applied to procedures involving the removal of blood from a donor or patient, separation of certain constituents from the removed blood, and subsequent return of the remaining blood constituents, including erythrocytes (red blood cells), to the donor or patient. Separation of plasma or plasma constituents from blood is known as "plasmapheresis"; separation of platelets, as "plateletpheresis"; and separation of leukocytes (white blood cells), as "leukapheresis". Although platelets or leukocytes can be removed separately, in many instances both are collected simultaneously for subsequent use in treatment (transfusion) of cancer patients whose blood-clotting and infection-fighting abilities have both been seriously compromised, either by the disease itself or by therapeutic procedures used to treat it.
Currently, most pheresis procedures involving removal of cellular constitutents are performed using continuous, semi-continuous or batchwise centrifugation. However, so-called "filtration leukapheresis" procedures have also been used to selectively remove certain white blood cell types from blood using a suitable sorbent, for example, as in the collection of granulocytes from blood by passing the blood over a suitable nylon-containing filter bed. U.S. Pat. No. 4,111,199 discloses a batchwise process for collection of leukocytes, the process comprising centrifuging whole blood to separate erythrocytes and leukocytes from supernatant plasma and platelets, resuspending the sedimented erythrocyte/leukocyte mixture in a saline solution containing an agent which selectively increases the sedimentation rate of erythrocytes by causing their aggregation, and then allowing the erythrocytes to separate at unit gravity from a supernatant containing nonsedimented leukocytes. The sedimentation of erythrocytes, with and without use of sediment-enhancing agents, is discussed by Cutts in "Cell Separation," Academic Press (1970), pp. 35-69, and continuous flow separators are discussed in "Leukocytes: Separation Collection and Transfusion," edited by Goldman and Lowenthal, Academic Press (1975), pp. 3-13 and 30-42.
It is an object of this invention to provide a continuous, noncentrifugal apparatus for efficiently separating whole blood, at unit gravity, into an erythrocyte-enriched plasma fraction and a leukocyte/platelet-enriched plasma fraction. Further separation of components of the leukocyte/platelet-enriched fraction can then be effected, if desired, by means which will be described hereinafter. Another object is to provide a continuous noncentrifugal apparatus which is efficient for collection of a leukocyte/platelet fraction containing minimal levels of erythrocytes so that significant depletion of a patient's or donor's erythrocyte level does not result from repeated phereses procedures to obtain leukocytes. Still another object is to provide such an apparatus which is mechanically simple, easy to fabricate and use, and provides a virtually enclosed system. Other objects will become apparent hereinafter.